As an increasing number of medical interventions call for multi-modality imaging, such as combined x-ray and magnetic resonance imaging (MRI), the design of x-ray systems must be adapted to allow for operation in a high magnetic field. In an MRI imaging environment, small magnitude high-frequency time-varying field gradients are superimposed to a large static field with a magnitude of several Tesla; usually only the static field is to be considered for shielding purposes when operating an x-ray system in the vicinity of an MRI system.
Other applications where compatibility of an x-ray imaging system with applied external magnetic fields is required include interventional radiology and cardiology, where a patient is positioned on a table within an operating and imaging region during the procedure. In a magnetic navigation procedure, a variable magnetic field is applied to guide the progress of a guide wire, guide catheter, sheath, or catheter, to enable easier navigation of such medical devices through the patient's vasculature. In the environment outside but nearby the navigation volume the magnetic fields are typically of a magnitude of a few tenths of a Tesla or smaller but vary throughout the procedure in an apparently unpredictable manner as dictated by the navigation needs. The direction and magnitude of the external field present around the navigation region and immersing the x-ray system can thus dynamically evolve in a time scale comparable to that of the x-ray imaging chain image acquisition sequence.
Normal operation of an x-ray radiographic or fluoroscopic system in a magnetic environment requires magnetic compatibility. In particular, the x-ray imaging chain, including the tube and detector, must include specific design considerations to enable high-quality robust imaging while being operated in a time and spatially variant magnetic field.
One of the key components to consider for magnetic compatibility is the x-ray source. In most imaging x-ray systems, an electron beam is accelerated from a cathode to a metal target anode through the application of a high-voltage potential difference; x-rays are produced by the subsequent deceleration of the electrons upon hitting the anode target material. In the presence of a magnetic field the beam electrons will experience a force (the Lorentz force) when a component of the magnetic field is perpendicular to the direction of electron motion. The Lorentz force deflects the electron beam and moves the electron focal spot (where the electrons hit the metal target) position on the anode; as a result the x-ray source location is shifted. Such x-ray source shifts are magnified by the x-ray system source-collimator-detector geometry and produce associated image shifts; accordingly the projection of a static object appears to be moving when imaged in a variable magnetic field. To the physician these types of artifactual image shifts are unacceptable.
Another source of image shift comes from the forces applied on the overall x-ray tube by the external magnetic and gravitational fields. In magnetic field magnitudes of 0.1 Tesla or less, the magnetic force is sufficient to induce flexing of the mechanical components that support the x-ray tube. The directions of the applied forces depend on the relative orientation of the x-ray tube and supporting structures with respect to the magnetic and gravitational fields. The resulting forces and torques on the image chain components can also create undesirable image shifts through differential flex behaviors of the x-ray tube and collimation sub-systems, and induce shifts in the relative geometry between the patient and the x-ray image chain. Such shifts can compromise the accuracy of three-dimensional (3D) spatial information derived from the x-ray projections and also can complicate or render unfeasible the task of registering the projection data to a previously acquired 3D data set.